Investigation of Blockage Upstream Box Coverage on the Scour Pattern in the Open Channel

Abstract

Under sub-critical flow conditions, the presence of blockage (solid wastes and weeds) upstream of box coverage and the scours pattern downstream of the coverage was explored in this study. The upstream blocking was simulated using 48 runs in an artificial trapezoidal cross-section with three square box coverages of side dimensions 8.80, 10.40, and 12.90 cm, four water flows of 2, 5, 8, and 11 L/s, and four blockage ratios of 0, 10, 20, and 30% relative to the coverage cross-section area. To estimate the scour hole characteristics, a 2.00 m long, 0.60 m bed width, and 0.30 m deep sand basin filled with D50 = 0.50 mm bed material was constructed directly downstream of the coverage outlet. In each run, the water level, velocity, and scoured hole parameters downstream of the coverage were measured. According to the analysis, the Non-blocked coverage has less scour depth and length than the partially blocked coverage, where the maximum scour depths and length of the Non- blocked coverage for cases 1, 2, and 3 at the discharge of 11 l/s were 73, 72, and 17 % respectively, and 77.56, 77.34, and 83.66 % respectively relative to the maximum scour depth and length of partially blocked coverage at the discharge of 11 l/s and blockage ratio 30 %. The depth and length of the scour hole downstream coverage are increased with the increment of the coverage's blockage ratio and discharge and are reversely proportional to the inlet area of coverage. Increased relative scour depth and length by 3.60 percent and 11.80 percent respectively, by increasing the downstream Froude number of flow (Frd) by 0.01 while the coverage’ relative wetted area (Ar) is constant. The study suggested applying coverage with a suitable area and water discharge, and protection techniques downstream of the coverage to reduce the influence of coverage and blockage on the open channel's hydraulic efficiency and the scour pattern downstream of the coverage. Also, install a trash rack upstream of the covering and remove aquatic weeds and solid wastes periodically upstream and inside the coverage.